go_1_9: skip flaky TestServerCancelsReadTimeoutWhenIdle

https://github.com/golang/go/issues/19608
https://hydra.nixos.org/build/64329767/nixlog/1
https://hydra.nixos.org/build/64244716/nixlog/1

Remove the patch for https://github.com/golang/go/issues/21559 because it is
skipped as flaky since Go 1.9.2.

pkgs/development/compilers/go/1.9.nix

@@ -118,11 +118,7 @@ stdenv.mkDerivation rec {
       ./ssl-cert-file-1.9.patch
       ./creds-test.patch
       ./remove-test-pie-1.9.patch
-
-      (fetchpatch {
-        url = "https://github.com/golang/go/commit/29415eb2b92e78481897c4161ba99f5b09fa6102.patch";
-        sha256 = "01jkm4b2dazzjnfla7rdd0w2clzplga3zza6ybpmkjkk3i4bp73d";
-      })
+      ./go-1.9-skip-flaky-19608.patch
     ];
 
   postPatch = optionalString stdenv.isDarwin ''

pkgs/development/compilers/go/go-1.9-skip-flaky-19608.patch

@@ -0,0 +1,12 @@
+diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
+index 7137599..43cca2e 100644
+--- a/src/net/http/serve_test.go
++++ b/src/net/http/serve_test.go
+@@ -5361,6 +5361,7 @@ func testServerKeepAlivesEnabled(t *testing.T, h2 bool) {
+ // the server's doing its 1-byte background read between requests,
+ // waiting for the connection to maybe close.
+ func TestServerCancelsReadTimeoutWhenIdle(t *testing.T) {
++	testenv.SkipFlaky(t, 19608)
+ 	setParallel(t)
+ 	defer afterTest(t)
+ 	const timeout = 250 * time.Millisecond